Auk, The, Oct 1999 by Sheldon, Frederick H, Winkler, David W
ANYONE WHO TRIES to identify a bird nest, without seeing the bird that constructed it, enters the realm of avian systematics. The attempt to determine the identity of the nest leads immediately to an effort to categorize the nest according to its overt features: Is it in a hole? Is it on a branch? If the nest lies in a hole: Where is the hole located? How big is it? How was the hole constructed? For the nest itself: What material is it made of? How is the material fitted together? How is the nest lined?
Such quests to identify nests depend ultimately upon evolution and the “nested” nature of nest architecture. Members of a group of closely related birds tend to build their nests based on a common architectural theme, and subgroups “nested” genealogically within larger groups tend to build nests that are variations on the larger theme. By examining a nest, we can quickly assess the general type of bird that constructed it (oriole, swallow) by the basic theme (pendant nest, mud nest). Then, by following a route of subthemes (e.g. shape, location, construction method, materials, etc.), we can whittle down the list of possible builders. The reason for the “nesting” of nest themes is that throughout evolutionary history, birds have met ecological challenges (e.g. changes in climate, predation, and competition) by adapting their nests to each new situation. These adaptations tended not to be revolutionary, because of genetic and selective constraints on the morphology and behaviors associated with nest building (Winkler and Sheldon 1994). Instead, they tended to be slight modifications on the main nesting theme. For bird systematists, who are scientists interested in understanding evolutionary patterns, this adaptive tinkering has provided invaluable clues to the history of avian life. It has created a hierarchy of nest types that, when deciphered, can shed light on the phylogenetic (genealogical) relationships of birds, and it has left an evidential trail of the interaction between genetics and ecology, the driving force of evolution. Thus, even a rudimentary consideration of the possible owners of an unidentified nest delves into the methods and logic of avian systematics and yields insights into bird evolution.
